The polyunsaturated fatty acid (FA), linolenic acid, when injected into the blood, crosses cerebral vessels by passing directly across the endothelial cell membrane. Some of the FA fills endothelial pits and vesicles and may represent that portion of the FA circulating in combination with serum albumin to which it is normally bound. Within endothelial, smooth muscle and adventitial cells, FA droplets penetrate the perinuclear cistern but are not visible within the nucleus. Droplets also enter choroid plexus epithelium from blood or CSF by passing directly across the cell membrane rather than in vesicles.